Abstract Social anxiety (SA) afflicts over 13% of the U.S. population and is one of the most prevalent forms of mental illness in both children and adults1,2. SA is associated with significant long-term impairment in academic, occupational, and social functioning3, and as such, SA is a major concern at both the individual and public health levels. The onset of SA peaks during adolescence4,5,6, making this a critical developmental period for identifying core underlying mechanisms that may inform early risk. To date, fear of negative evaluation (FNE) is one of the most well-documented and widely accepted core components of SA7,8. However, in recent years, a small literature has emerged suggesting that intolerance of uncertainty is as strong a predictor of SA as FNE9,10,11. This growing literature signals the need for greater examination of sensitivity to uncertainty as a core vulnerability in SA, and highlights the need to integrate unpredictability into investigations of social evaluation. Despite this, only one study has directly investigated the impact of unpredictable social evaluative feedback on peer ratings11, and no study has experimentally manipulated and examined the impact of unpredictability on neural or physiological responding to social threat. In the proposed training and research grant, we will develop and utilize a novel task that will directly test our heuristic model of SA that highlights the role of unpredictability of social threat as a core element of SA. Our novel task will examine the physiological and neural correlates of stimulus processing in anticipation of predictable and unpredictable social evaluative feedback. The proposed study has three specific aims: (1) to leverage a novel task to assess anticipation of predictable and unpredictable social evaluation in adolescents across multiple domains of measurement (i.e., startle, event- related potentials (ERPs), and self-report), (2) to examine the association of SA symptoms with lab-based measures of anticipation of unpredictable social evaluation, and (3) to obtain training in ecological momentary assessment (EMA) to explore the relationship between our lab-based measures of sensitivity to unpredictable social feedback and real-world functioning and anticipatory anxiety. One hundred adolescents from a recently- NIMH-funded longitudinal R01 will be recruited for the study. Under the guidance of Dr. Hajcak, eye-blink startle and scalp-recorded ERPs will be simultaneously recorded during our novel lab-based task, and real- world functioning will be examined using EMA in a subsample of participants (n = 50). The current training proposal will allow me to integrate training from co-mentors who have expertise in the study of affective disorders using psychophysiological measures (Dr. Hajcak) and in models of affective disorders in developmental populations (Dr. Klein). The integration of training and mentorship received from Dr. Hajcak on psychophysiological methods and from Dr. Klein in developmental psychopathology will propel me towards my goal of becoming a developmental clinical psychophysiologist. Furthermore, experts will provide consultation in the role of sensitivity to unpredictability in social anxiety (Drs. Jarcho and Pine), collection of psychophysiological measures in developmental samples (Dr. Pine) and training in the use of EMA in adolescent populations (Dr. Silk).